In general, accurate measurement of a temperature of a substrate (e.g., a semiconductor wafer) to be processed by a substrate processing apparatus is very important in controlling the shape and property of a film, a hole and the like formed thereon by various processes such as film forming, etching and the like. For this reason, various methods of measuring a temperature of a semiconductor wafer, including a measurement method using a resistance thermometer or a fluorescent thermometer for measuring a temperature of a rear surface of a substrate and the like, have been performed.
In recent years, a temperature measurement technology using a low coherence interferometer capable of directly measuring a temperature of a wafer, which was difficult by the above method, has been proposed.
Further, in the temperature measurement technology using the low coherence interferometer as described above, there is proposed a method (see, for example, Patent Document 1) including the steps of: dividing a light beam originated from a light source into a measurement beam for temperature measurement and a reference beam by using a first splitter; dividing the measurement beam into n number of measurement beams by using a second splitter to irradiate the n number of measurement beams onto n number of measurement points; measuring an interference of reflected beams of the n number of measurement beams and a reflected beam of the reference beam reflected by a reference beam reflector; and measuring temperatures of a plurality of measurement points at the same time. Using such a technology, it is possible to simultaneously measure temperatures of a plurality of measurement points with simple configuration.    (Patent Document 1) Japanese Patent Application Publication No. 2006-112826